doc: Clarified Mac OS X set up instructions

Mac OS X set up instructions were missing some important details that
prevent a new user to set up Zephyr development environment on a not
yet configured system.

JIRA: ZEP-616

Change-Id: I0890ec5364a0ce21e38ba4101d285fc8e46a6bd8
Signed-off-by: Genaro Saucedo Tejada <genaro.saucedo.tejada@intel.com>